See everyone gets nervous it may be the most popular speaker of the school or the teacher itself so its totally normal to feel that way but what makes them speak even without looking nervous is that, they are confident on what they are going to speak the answer maybe lame or wrong but the way you present it matters the most, you may not know the answer of the question asked to you in front of the class so just let out anything you know on that topic just anything for eg. if you were asked to talk on the topic 'is youth able to change the condition of india" so basically you have no idea on what to say because you are nervous or you think that you are going to make a fool out yourself just erase that thought out of your mind and  say whatever comes in your mind and trust me with the first line itself your brain will have a flow of ideas(human physcology) be confident if not just fake confidence until you feel it and remember no one is going to kill you if have the wrong answer, even Einstein asked questions!!  99% of people overlook the mistakes if the speaker is confident on his/ her point and make sure to use good vocabulary,and before you speak take 10 deep breathes it will calm your nerves and oxygen intake and count till 50 backwards(if you have time)

good luck :)